This project has succesfully led to the development of the first in vitro model system in which the biologic behavior of dormant or proliferative metastatic cells can be correlated to their growth properties in our in vitro system. This has important implications in the field since now it is possible to study how the extracellular matrix influences the transition from quiescent to proliferative growth. We have demonstrated that the composition of the matrix can determine whether dormant cells remain quiescent. Our studies have shown that one component of the extracellular matrix, fibronectin, can activate the dormant cells to proliferate through the engagement of integrin beta 1, a key membrane protein that mediates signaling from the extracellular matrix to the cell nucleus leading to profound molecular changes. We have further demonstrated that another protein often found in tumors with poor prognosis or a high propensity to metastasize can activate the same pathway. Our molecular studies have dissected this molecular pathway in detail and we have identified potential targets that may inhibit this process. Further pre-clinical studies are ongoing to determine whether the dormant to proliferative can be inhibited with specific drugs. A highly novel finding of this work was that the dormant to proliferative switch requires a major change in the architecture of the cytoskeleton. If this process of actin stress fiber is inhibited by several methods, the cells remain dormant. This provides an additional set of molecular targets to test as inhibitors of the dormant-to-proliferative switch that may reduce the risk of tumor recurrence. Initial studies have taken advantage of our novel in vitro model system to identify key mediators of the signaling pathway. These findings are further validated using in vivo model systems to confirm the role of targets we have identified. We are currently testing in our animal model a drug that is currently in clinical trials which may have a significant effect in inhibiting the growth of the dormant tumor cells in vivo. Positive results of these studies would have important translational implications. We are developing new novel model systems in which we are able to alter the tumor microenvironment in mice to determine how such changes affect the growth of dormant tumor cells. Since changes in the microenvironment are associated with significant increases in risk of breast cancer mortality and tumor recurrence, these studies will provide very relevant models to study these processes that cannot be studied in human patients. The study of tumor cell dormancy is an area that has been minimally explored but accounts for a major cause of death from recurrent cancer. Therefore, these studies will provide important insights into this area that is highly relevant to the mission of NCI. Our work focused on BMI1 has demonstrated that the expression of this gene in combination with another oncogene can dramatically alter the aggressive nature of breast cancer cells and result in a much higher incidence of metastatic spread. Ongoing gene expression and proteomic studies will identify mechanisms that are involved in these biologic changes and lead to new translational opportunities to inhibit tumor progression and metastases.